This application claims the priority of German Application No. 199 21 668-1 filed May 11, 1999, which is incorporated herein by reference.
The invention relates to a method of moving a turntable in an apparatus for molding and punching out containers of the generic type described in the main claim, and a device for executing the method.
From DE 33 46 628 C2, it is known to configure the molding table of an apparatus for deep-drawing containers from thermoplastic plastic foil as a turntable. This permits the lower molding-tool half to be pivoted, with the molded, punched-out containers, into a discharge position, in which the containers are ejected from the deep-drawing molds, preferably directly into stacking grooves. A vertical downward stroke prior to the pivoting of the turntable into the stacking position is necessary for two reasons. First, a tool edge of the lower tool half would collide with the upper, stationary tool half. Secondly, the punching absolutely must take place in a vertical movement of the two tool halves for the two tools to be guided together.
The vertical movement of the turntable is effected by cam disks, with one cam disk for the upward movement and one cam disk for the downward movement, as well as a toggle lever and a lever rod assembly. The two cam disks must be exactly matched to one another, which entails complex calculations, costly manufacturing and precise assembly. The turntable is pivoted by means of a third cam disk and a bracket mounted to the molding table. The pivot point of the turntable also serves as the point of articulation of the toggle lever, and is seated in a guided bearing. This means that the pivot point, and therefore the bearing units, execute a linear up-and-down movement during a stroke. In terms of energy, the toggle-gear drive must exert corresponding work during each stroke, and this work must be cancelled out during the downward movement. This process generates accelerating and braking forces, which act on the components and impact their wear behavior.
Another problem associated with pivoting is accelerating and braking the turntable. Particularly in FIG. 3 of DE 33 46 628 C2, the drive for the pivoting movement and the components that must absorb the forces of the moved masses are visible. An objective is to position the pivot point of the turntable in the center of gravity of the unit comprising the turntable and the lower tool half. This is not always the case, however, because the molding tool is exchangeable, and has a different embodiment depending on the shape and the number of containers to be produced per batch. Its weight also varies, which influences the position of the center of gravity. Yet, even if the table is pivoted in the center of gravity, high dynamic forces must be absorbed in the end positions. The highest forces occur when the turntable is pivoted into the stacking position, because then the table""s own weight must likewise be supported. Especially in this stacking position, the turntable/tool half unit tends to vibrate, causing tremendous wear of the components that must absorb these forces.
In terms of the method, in this known apparatus the pivot point is displaced vertically, with a fixed distance between the pivot point and the center of gravity. The magnitude of the fixed distance is a function of the tool; a distance of zero is desirable. In this method, high forces occur in the end positions in the driving components, thereby limiting the number of strokes of the apparatus. The apparatus is costly to produce, and experiences significant wear when a higher stroke number is employed.
DE 197 10 475 A1 discloses an apparatus having a turntable, in which the turntable is moved vertically and pivoted over curved paths. The pin Pos. 4, and thus the pivot point, execute a vertical movement. The turntable suspended in this pivot point has a center of gravity that is spaced from the pivot point by a fixed distance, which does not change during a stroke. The distance can be zero, and is a function of the tool weight. Therefore, in principle, the same problems occur here as in the method according to DE 33 46 628 C2.
High forces occur in the end positions, particularly during tipping, limiting the maximum cycle number of the apparatus. The apparatus has complicated curved paths. The principal problem of guiding a curve roller in a groove is that the direction of rotation of the curve roller is a function of the side of the groove with which it is in contact. Because the contact side alternates, the roller alternatingly rotates in both directions; a change in the direction of rotation causes a high degree of wear to the groove wall and the curved roller, because there is no rolling friction during this time. To avoid this problem, a plurality of curve rollers can be built into each groove (FIG. 2), but this solution is expensive and is associated with rigidity-related problems, because the roller spacing must be changed depending on the curved path. These rollers are therefore resilient, which is, unfortunately, conducive to vibrations of the molding table, because the curve rollers can yield corresponding to the forces.
It is known from CA 2 240 428 A1 to move a turntable over three very complicated curved paths and three curve rollers. No definable central pivot point is present in such a spatial displacement of a table; the direction of movement of the center of gravity of the molding table extends in a specific path as a function of the shape of the curved paths. This pivoting method requires a very complicated, costly apparatus with three curved paths that are exactly matched to one another and are disposed on both sides of the turntable. In this redundant system, the risk exists that the curve rollers will jam. The system is also affected by the problem of the changing direction of rotation of the curved paths depending on the contact side, which changes constantly, resulting in wear. High forces act on the drive elements and the curve rollers during the braking of the pivoting movement, primarily in the approach to the stacking position.
It is the object of the invention to execute the method of pivoting the turntable such that relatively-small gravitational forces occur during the approach to the end positions of the pivoting movement, and a fast pivoting movement can occur, so the apparatus can be operated with a high number of strokes. The method is intended to require only an apparatus with a simple design, without expensive, complicated curved paths. The method is intended to preclude wear-related problems due to the displacement of a curve roller in the groove of a curved path with correspondingly changing directions of rotation.
According to the invention, the above object generally is accomplished by the selection of a stationary pivot point for the turntable, and the altering of the distance of the turntable/molding-tool half unit during pivoting. The distance is greater in the end positions of the movement, and is at a minimum, or zero, at one-half of the pivoting angle. The reduction in the distance at the beginning of the pivoting movement additionally accelerates the turntable, which increases the rotating speed. Correspondingly, the increase in the distance during the braking phase as the end positions are approached effects an additional reduction in the rotating speed, without the components being forced to absorb associated forces. The braking and accelerating forces to be absorbed by the components are reduced correspondingly, which permits a high stroke number for the apparatus with low wear. It is advantageous if the distance between the center of gravity and the pivoting point changes continuously during pivoting, and a minimumxe2x80x94possibly to the value of zeroxe2x80x94is attained at one-half of the pivoting angle.
The apparatus for executing the method is embodied to include a stationary pivot point, to which the turntable is displaced, with the molding-tool half secured to it, during pivoting such that the center of gravity is moved toward or away from the pivot point. With this design, the position of the bearings for the pivot point need not be changed, which reduces the overall masses to be moved.
In a modification of the apparatus, laterally-pivoting supports support the turntable in the closed position for the molding and punching process. This permits a relatively-lightweight design of the turntable, yet a stable construction that resists bending during molding and punching.